But now some progressives have him in their sights, believing the Kentucky Republican's opposition to gun reform offers Democrats a foothold in McConnell's 2014 re-election race. Case in point: The Progressive Change Campaign Committee launched an ad campaign on Tuesday attacking McConnell's gun stance.

"Sen. Mitch McConnell has taken thousands of dollars from gun manufacturers, and he opposes common-sense reforms. Sen. McConnell, whose side are you on?" the ad's narrator, Kentucky voter Rodney Kendrick, asks in the ad as he holds his grandson on his lap.

The ad launched with a $27,700 initial buy on broadcast and cable in Louisville and Lexington and on cable in Washington, D.C., according to the committee.

The PCCC said data it commissioned from the Democratic firm Public Policy Polling suggests Kentucky voters back some of the reforms for which McConnell has not issued public support, including universal background checks and an assault weapons ban.

Eighty-two percent of likely Kentucky voters surveyed by the firm stated support for universal background checks. Thirteen percent were opposed. That survey's margin of error was plus or minus 3.8 percentage points.

The firm also asked about an assault weapons ban and found wide support.

But McConnell campaign manager Jesse Benton on Tuesday dismissed the ad and the group behind it, saying in a statement: "Sen. McConnell fully anticipated that protecting Kentucky from President Obama's gun-control agenda would result in a flurry of attack ads from left-wing groups. It's not a secret to Kentuckians that Sen. McConnell is a stalwart supporter of their Second Amendment rights and George Soros-funded commercials aren't going to change that."

Actress Ashley Judd has expressed interest in running for the Democratic nomination for the Kentucky U.S. Senate seat, but it remains unclear if she is serious about pursuing a campaign.